Published German patent application 3,929,025 (corresponding to U.S. patent application Ser. no. 07/822,146 filed on Jan. 17, 1992, now abandoned) discloses a carburetor for an internal combustion engine having a throttle flap mounted in the intake channel. At least one idle outlet bore opens into the region of the throttle flap and a main nozzle opens forward of the throttle flap viewed in the intake air flow direction. The main nozzle is connected via a fuel channel to a fuel-filled control chamber and the idle chamber is connected to the control chamber via a channel having a fixed throttle disposed therein. An opening having a variable cross section is disposed between the control chamber and the main nozzle. The cross section is changeable by means of a control screw accessible externally. The fuel quantity metered per unit of time to the intake channel is determined with the aid of this control screw.
U.S. Pat. 3,170,006 discloses a membrane carburetor having an independent idle system wherein a main nozzle is connected via two fuel-conducting paths to the fuel-filled control chamber. An opening is provided in a closure plate which determines a first fixed amount of fuel flowing to the main nozzle. Parallel thereto, a bore is provided in the housing of the carburetor which coacts with an adjustable needle and functions as an adjustable throttle. Needle valves of this kind cannot, however, be produced with any desired reduced maximum cross section so that a relatively large amount of fuel is supplied to the carburetor when the adjustable throttle is fully open.
In known arrangements, the maximum and/or minimum permissible fuel quantities are adjusted with the engine running and normal operating temperature. However, this is associated with a corresponding expenditure of effort. If the operator later adjusts the control screw during operation, then the previously adjusted maximum fuel quantity is no longer applicable and the carburetor must again be adjusted with a new measurement.
Furthermore, between the maximum open position and the closed position of the adjustable throttle, a cross sectional change is provided which is so large that the total passthrough quantity of fuel is doubled or increased therebeyond.